It has been found that most of the energy of a seismic event is absorbed and dissipated, in a building having a structural steel framework, in the beam-to-column connections of the building.
The prior art teaches numerous connections of beams to columns. Experience in recent earthquakes has taught that such traditional connections must be improved.
Previously, the most common beam-to-column connection has been one in which the beam has the ends of its top and bottom flanges welded to one flange, or face, of the column by large, highly-restrained, full-penetration, single-bevel groove welds. Vertical loads, that is, the weight of the floors and on the floors, are commonly carried by vertical shear tabs. Each such shear tab is vertically disposed and is welded to the face of the column and bolted or welded to the web of the beam, at the end of the beam at the column, using high-strength bolts.
There has been partial or complete failure of the highly-restrained welds between the beam flange and the column flange, either by a crack in the weld itself or a crack along the heat affected zone of the column flange, pulling a divot of column steel from the face of the column flange. The origination of the crack is normally at the narrow root of the groove weld profile, which is inherently subject to slag inclusions during the field welding process. These inclusions act as stress risers that initiate cracking during the impactive load from an earthquake.
Stress risers are also created by the backer bar used to bridge the root gap before making the weld. The backer bar is commonly tack welded in place below each beam flange and not removed. In addition, these failures between the beam flange and column flange have resulted in shear failure of the high strength bolts connecting the shear tabs to the web of the beam for the support of the gravity loads.
In other instances, the crack again originates at the root of the groove weld, but enters the column flange and propagates through the full thickness and width of the flange and into the column web.
Subsequent attempts by the building industry to improve beam-to-column connections still rely on post-yield straining of large, highly-restrained, full-penetration, single-bevel groove welds performed under field conditions. Such highly-restrained welds do not provide a reliable mechanism for dissipation of earthquake energy, or other large forces, and can lead to brittle fracture of the weld and the column. Such brittle fracture is in violation of the moment-resisting design philosophy of the Uniform Building Code.
It is desirable to achieve greater strength in such beam-to-column connections in order to make buildings less vulnerable to earthquakes, explosions, tornadoes or other large scale, damaging occurrences. The invention herein is particularly useful in upgrading and strengthening pre-engineered steel frame buildings for improved blast resistance.
In the case of earthquakes, greater strength is particularly desirable in resisting sizeable moments in both the lateral and the vertical plane. That is, the beams in a building, in an earthquake, are caused to move both horizontally and vertically, placing severe stresses on the locations where the beams are connected to the columns.
Engineering analysis, design and testing have determined that prior steel frame techniques can be substantially improved by strengthening the beam-to-column connection in a way which better resists and withstands the sizeable moments which are placed upon the beam.
It is a goal, therefore, to increase lateral and vertical stability as well increase the vertical load-carrying capability. The invention herein provides such capability, providing both a lateral and vertical moment resisting connection, and increased, vertical load-carrying capability. Further, the invention complies with the emergency code provisions issued by the International Conference of Building Officials.
Consequently, the improved design of the invention is capable of carrying greater loads and capable of withstanding greater earthquakes and other calamities which may place extreme strain on a structure.
Another feature of the invention is that it is cost-effective. By providing stronger beam-to-column connections, lighter steel beams and columns can be used, while still providing greater strength in the beam-to-column connections and, also, in the overall structure of the building, compared to prior structures.
The beam-to-column connection invention herein is may be made in the shop under controlled conditions and placed in a retrofit construction. Shop fabrication provides for better quality construction of a beam-to-column connection by reason of better control of the manufacturing process and easier access to and handling of all parts of the connection. The invention effectively makes use of fillet welds, which are better made under shop conditions, although it can suitably be made in the field. Splice plates are commonly used in the field to insert column sections and beam sections in their selected place in a structure. Such splice connections are located at structural points of reduced flexural stress. That is, the splice connections are located at some distance from the beam-to-column connection.
However, the invention herein is particularly effective when used in field retrofit modification wherein beams and columns require strengthening in place and wherein beam-to-column connections are to be strengthened in place, in structures having floors, walls and roofs already in place and attached to the beams and columns.
However, the invention herein may be used in new construction and may be constructed in place and on site.